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All you need to know about heat waves

Future heat waves will be more extreme in temperature, happen more frequently, last longer and cover more of the Earth’s surface.

Excessive heat is especially harmful to elderly women, particularly those who live alone.

Peter Macdiarmid / Getty Images

Heat waves kill more people in Australia than bushfires. By 2040, in each calendar year, heat waves will sweep across 20% of the land area of our planet. (That’s a massive increase from just 1% in the 1960s.)

Most Australians would remember the terrible 2009 Black Saturday disaster in Victoria. Flames, heat and smoke from those horrendous bush fires killed 173 people. But what most Australians don’t realise is that the crippling heat waves around Black Saturday killed 374 people. That’s more than twice as many people.

Overall, heat waves have killed more Australians than all other natural hazards combined – 55% of all such deaths. More than 4,500 Australians have died from heat waves since the year 1900.

In the European heat wave of 2003, some 50,000 to 70,000 people died between June and August. The Russian heat wave of 2010 killed around 55,000 people.

Thanks to global warming, future heat waves will be more extreme in temperature, happen more frequently, last longer and cover more of the Earth’s surface.

What exactly is a heat wave?

Confusingly, the definition varies depending on the country. Sometimes it can vary from one state to another within a country – such as in the US.

One widely accepted definition comes from the World Meteorological Organisation (WMO). It starts by setting the baseline as the 30 years between 1961 and 1990. Then, for the location you’re interested in, pick one day of the year, such as 27 January. Each of those “27 January” days across the 30 years will have a minimum and a maximum temperature (usually after midnight, and after midday).

Add the 30 maximum temperatures, divide by 30, and then you have the average maximum temperature for that particular day and location. Do the same for the next four days to give you the average maximum temperature for five days in a row.

According to the WMO, a heat wave happens when you have five days in a row that each have a daily maximum temperature 5 °C or more higher than the average maximum temperature.

What causes a heat wave?

Basically, a heat wave occurs when a high-pressure system in the atmosphere, instead of moving across the landscape, stays stuck in one location – for days or even weeks.

But in the mega heat waves that killed tens of thousands of people in Europe and Russia, there was another factor. These heat waves were made worse by a vicious positive feedback loop between ultra-dry soil and unexpectedly powerful high-pressure systems in the lowest level of the atmosphere.

This combination trapped the heat. The trapped heat couldn’t dissipate overnight – so the next morning started as hot as the previous afternoon. The cycle intensified with each passing day. Ultimately, it created a thick blanket of hot air, four kilometres thick.

How can you tell if a specific death is caused by a heat wave?

Mostly, you can’t – directly. There are many factors involved. How well you tolerate heat depends on what temperatures you are used to, your age and general health, your home’s architecture and its location, and so on.

So the heat wave that kills one person might not kill their neighbour. And when you do an autopsy, there is no specific pathology in the corpse that implicates a heat wave as the cause of death.

But you can tell, indirectly, when heat waves cause deaths. You know that something very bad is happening when the dead bodies start to pile up in the morgue.

In the heat waves of Europe in 2003, Victoria in 2009, Russia in 2010 and Victoria again in 2014, morgues had the metaphorical “No Vacancy” signs up. There was simply no more room to store the dead bodies coming in. Overflow had to be stored in mortuaries, universities and funeral parlours. In Paris in 2003, the corpses of most of the 15,000 heat-related victims had to be stored temporarily in a refrigerated warehouse outside the city.

Then, after the heat wave has passed, you call in the statisticians to work out how many people it killed. They compare the number of deaths during the heat wave with the number of deaths over the same time period in previous years.

In Australia, the most lethal day for a heat wave is 27 January, the day after Australia Day.

What exactly is it that kills somebody in a heat wave?

Amazingly, we still don’t fully understand what goes on. In Paris alone in 2003, some 15,000 people died. In this case, they were overwhelmingly elderly women, living alone and in the upper levels of walk-up apartments.

Excessive heat seems to be especially harmful to the very young and the very old, and also to those with chronic diseases and mental illnesses. Related risk factors include being obese, very malnourished and very unfit.

Another factor is drugs – both legal and illegal. Dehydration combined with alcohol consumption makes the situation worse.

When the electrical power grid crashes, the loss of air-conditioning in poorly designed houses can be fatal. Another factor in Europe is that the houses are generally designed to keep the heat in, not out.